The present invention relates in general to material handling vehicles and, more particularly, to a pallet clamping device associated with forks of such vehicles for selectively clamping or grabbing a center rib or stringer of a pallet supported by the forks so that the pallet is effectively locked to the forks and cannot move off the forks.
A variety of pallet clamps and other pallet locking devices are known in the prior art. In U.S. Pat. No. 2,256,454, a lift truck includes a load pushing frame mounted above its forks and a pair of pallet grippers. Both the grippers and the load pushing frame are actuated to push a piled load of bags off a pallet upon which the load is stacked while the grippers engage a center rib of the pallet to retain the pallet on the truck as the load is pushed off the pallet. The pallet grippers, best shown in FIGS. 3 and 4, include a pair of grab hooks that are activated by cam plates and a spread fork by operation of a solenoid or by a double acting hydraulic cylinder. The grab hooks engage hook bars added to the center rib of pallets being used with the truck. An alternate pallet gripper, shown in FIG. 7, includes a lug integral with a fork and a socket formed in the underside of the top surface of a pallet being used with the truck.
Another currently popular form of pallet clamping device includes passive locking clamps wherein either single cam action forces or double cam action forces resulting from movement of a pallet off the forks wedge the cam or cams into a center rib or stringer of the pallet to prevent the pallet from moving off the forks. The structure and operation of such a pallet clamp is illustrated in FIGS. 1-4 wherein a foot pedal 10 controls extension of a cable 12 to move wedge cams 14 to an open position against the force of torsion springs 16 mounted to move the wedge cams 14 toward a closed position. As shown in FIGS. 2-4, the wedge cams 14 accommodate a variety of widths of center ribs or stringers 17 as a pallet is engaged. Once a stringer has been moved into the wedge cams 14, into which they can freely move, its exit is blocked by the cam action of the wedge cams 14 to firmly grip the stringer.
Yet another form of pallet clamping device is disclosed in U.S. Pat. No. 6,767,175 wherein lock bars positioned between forks of an order picking truck have tapered portions positioned on upper faces of a top end of the lock bars as shown in FIGS. 3-5. When the forks are inserted into a pallet, if the tapered portions come into contact with an end face of an upper deck board of the pallet, the lock bars rotate downward. If not, the lock bars rotate downward upon lifting of the pallet. When a landing sensor is activated by a magnet, see FIGS. 3 and 7a-7d, the lock bars are immediately driven and the locking operation is started. The ends of the lock bars push or come into pressure contact with an upper face of a lower deck board of the pallet, and the pallet is locked. A damper provides a resilient force to ensure that the lock bars remain engaged in spite of oscillations due to movement of the truck.
While known pallet clamping devices each have their own advantages and disadvantages, there is an ongoing need to advance the state of the art and provide new pallet clamping devices that offer alternates to existing designs and produce clamping characteristics that facilitate material handling operations performed by trucks equipped with the new clamping devices.